A printed circuit board widely used in an electronic implement comprises, for example, a synthetic resin sheet and a sheet of copper foil bonded onto each of the both surfaces of the synthetic resin board through a glass fiber sheet impregnated with an epoxy resin (hereinafter referred to as the "prepreg").
A typical method for manufacturing a printed circuit board is described with reference to FIG. 1. FIG. 1 is a schematic descriptive view illustrating a method for simultaneously manufacturing a plurality of printed circuit boards by the use of a hot press.
First, a sheet of copper foil 9a, a prepreg 8a, a synthetic resin sheet 7, another prepreg 8b and another sheet of copper foil 9b are placed one on the top of the other in this order to prepare a set of laminated sheets 6a. Another set of laminated sheets 6b is prepared in the same manner as described above, and thus a plurality of sets of laminated sheets 6a and 6b are prepared.
The above-mentioned plurality of sets of laminated sheets 6a and 6b are arranged between a pair of pressing portions of a hot press 12, as shown in FIG. 1, i.e., between a lower pressing portion 14 provided on a stand 13, and an upper pressing portion 15 arranged upward the lower pressing portion 14 at a prescribed distance therebetween so as to face same, and are pressed at a prescribed temperature. For the purpose of uniformly pressing each of the plurality of sets of laminated sheets 6a and 6b, a pair of pressing plates made of steel, i.e., a lower pressing plate 10a and an upper pressing plate 10b, each having a thickness of about 5 to 10 mm, and a plurality of separating sheets made of steel, i.e., a lower separating sheet 11a, an intermediate separating sheet 11b and a lower separating sheet 11c, each having a thickness of about 1 to 2 mm, are employed.
A block is thus prepared by placing, from down to top in the following order, on the top of the other, the lower pressing plate 10a, the lower separating sheet 11a, the set of laminated sheets 6a, the intermediate separating sheet 11b, the other set of laminated sheets 6b, the upper separating sheet 11c and the upper pressing plate 10b. A plurality of through-holes 16, 17 are provided at prescribed points on the thus prepared block, and mutual slip between the components of the block is prevented by inserting pins (not shown) into these through-holes 16, 17.
The above-mentioned block is mounted on the lower pressing portion 14 equipped with a heater, provided on the stand 13 of the hot press 12. The hot press 12 is actuated in this state to cause the upper pressing portion 15 equipped with a heater to go down and thus to apply a pressure on the block at a prescribed temperature. As a result, the epoxy resin impregnated in each of the prepreg 8a and the other prepreg 8b is melted, and this causes the sheet of copper foil 9a and the other sheet of copper foil 9b to be respectively bonded onto the both surfaces of the synthetic resin sheet 7 in the set of laminated sheets 6a, thus forming a printed circuit board. A sheet of copper foil and another sheet of copper foil are also respectively bonded onto the both surfaces of a synthetic resin sheet in the other set of laminated sheets 6b in the same manner as described above, thus forming another printed circuit board.
After the completion of bonding of the sheet of copper foil 9a and the other sheet of copper foil 9b onto the both surfaces of the synthetic resin sheet 7, the hot press 12 is actuated to cause the upper pressing portion 15 to go up to release the pressure on the block, and thus to take out the block from the hot press 12. Then, the above-mentioned pins are removed from the block thus taken out, and the lower pressing plate 10a, the lower separating sheet 11a, the intermediate separating sheet 11b, the upper separating sheet 11c and the upper pressing plate 10b are removed from the block, thereby, simultaneously manufacturing a plurality of printed circuit boards.
The above-mentioned method for manufacturing a printed circuit board has the following problems; When applying a pressure onto the block by the use of the hot press 12 as described above, part of the melted epoxy resin between the synthetic resin sheet 7, the sheet of copper foil 9a and the other sheet of copper foil 9b is forced out from the edges of these sheets and from the through-holes 16, 17 inserted with the above-mentioned pins, and adheres onto the surfaces of the lower pressing plate 10a, the lower separating sheet 11a, the intermediate separating sheet 11b, the upper separating sheet 11c and the upper pressing plate 10b.
Adhesion of the epoxy resin onto the surfaces of the lower pressing plate 10a, the lower separating sheet 11a, the intermediate separating sheet 11b, the upper separating sheet 11c and the upper pressing plate 10b causes the following demerits:
As a means to solve the above-mentioned problems, there is known a method for preventing adhesion of epoxy resin onto the surfaces of the lower separating sheet 11a, the intermediate separating sheet 11b and the upper separating sheet 11c by covering the surfaces of these separating sheets 11a, 11b and 11c with a fluorocarbon polymer film (hereinafter referred to as the "prior art 1").
However, the prior art 1 has the following defects:
Japanese Utility Model Publication No. 60-10,816 dated Apr. 12, 1985 discloses a mold for forming plastics, rubber or glass, which has on the inner surface thereof a plating layer in which fluorocarbon polymer particles are uniformly dispersed (hereinafter referred to as the "prior art 2").
The prior art 2 further discloses the following:
The lower separating sheet 11a, the intermediate separating sheet 11b and the upper separating onto the block by the use of the above-mentioned hot press 12, are required to have a Vickers hardness of at least 500 Hv. In the prior art 2, however, no regard is given to the effect of both the phosphorus content in the plating layer and the content of the fluorocarbon polymer particles in the plating layer, exerted on both hardness and strippability of the plating layer, in the case where the plating layer comprises a nickel-phosphorus alloy. In the prior art 2, as a result, it is not ensured that the plating layer is excellent in strippability and has a Vickers hardness of at least 500 Hv.
It is not therefore possible to apply the technical idea of the prior art 2 to the lower separating sheet 11a, the intermediate separating sheet 11a, the intermediate separating sheet 11b and the upper separating sheet 11c, which are used when applying the pressure onto the block by the use of the hot press 12. Even when the technical idea of the prior art 2 is applied to the lower separating sheet 11a, the intermediate separating sheet 11b and the upper separating sheet 11c to form a plating layer, in which the fluorocarbon polymer particles are uniformly dispersed, on the surfaces of these separating sheets 11a, 11b and 11c, it is not ensured that the thus obtained separating sheets simultaneously satisfy the following two conditions: (1) The separating sheets are excellent in strippability, i.e., they have a strippability sufficient to permit easy removal of the epoxy resin adhering onto the surfaces of the separating sheets by means of a metal spatula or a knife; and (2) The separating sheets have a sufficient hardness, i.e., a Vickers hardness of at least 500 Hv.
Therefore, if the thus obtained separating sheets are used when applying the pressure onto the block by the use of the hot press 12, the following problems may be caused: (a) It is difficult to completely remove the epoxy resin adhering onto the surfaces of the separating sheets even with the use of a metal spatula or a knife; and (b) During the above-mentioned removing operation of the epoxy resin, the metal spatula or the knife may cause deep flaws in the plating layer, and moreover, may cause flaws, not only in the plating layer, but also in the substrate itself, and such flaws in the plating layer and the substrate degrade the quality of the printed circuit board, as described above.
Under such circumstances, there is a strong demand for the development of a composite nickel-phosphorus alloy plated metal sheet excellent in strippability and having a Vickers hardness of at least 500 Hv, which is most suitable as a separating sheet to be used when manufacturing a printed circuit board by the use of a hot press, but such a composite nickel-phosphorus alloy plated metal sheet and a method for manufacturing same have not as yet been proposed.